Glad to hear 1900Mhz works.
I also purchased a 4G Exhibit II back in January with the $30/mo plan but since it can't get the more common 3G and there aren't very many 4G towers around it turned out to be mostly worthless and couldn't get data anywhere unless it was on wi-fi so I gave up after a while and got a Galaxy S-2. I was wondering if it would work on the new 3G 1900Mhz service, if it does that might give it some useful life after all so it won't be a total waste of money.
Mine was already at 2.3.5 and I updated to 2.3.6 but couldn't really tell any difference. I also rooted it and updated to 4.0.3 to see if that would give it 3G capability and while Android 4 runs OK data still won't work anywhere around here.
BTW, if you miss paying your bill on time your service might revert back to the regular plan (I heard of it happening to someone) but you can avoid that by setting up auto pay using a credit card so it gets paid automatically each month.